From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <464B014B.20109@yahoo.com.au> Date: Wed, 16 May 2007 23:04:11 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2 References: <20070318233008.GA32597093@melbourne.sgi.com> <18993.1179310769@redhat.com> <1179317360.2859.225.camel@shinybook.infradead.org> <20070516125341.GS26766@think.oraclecorp.com> In-Reply-To: <20070516125341.GS26766@think.oraclecorp.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Chris Mason Cc: David Woodhouse , David Howells , David Chinner , lkml , linux-mm , linux-fsdevel List-ID: Chris Mason wrote: > On Wed, May 16, 2007 at 08:09:19PM +0800, David Woodhouse wrote: > >>On Wed, 2007-05-16 at 11:19 +0100, David Howells wrote: >> >>>The start and end points passed to block_prepare_write() delimit the region of >>>the page that is going to be modified. This means that prepare_write() >>>doesn't need to fill it in if the page is not up to date. >> >>Really? Is it _really_ going to be modified? Even if the pointer >>userspace gave to write() is bogus, and is going to fault half-way >>through the copy_from_user()? > > > This is why there are so many variations on copy_from_user that zero on > faults. One way or another, the prepare_write/commit_write pair are > responsible for filling it in. I'll add to David's question about David's comment on David's patch, yes it will be modified but in that case it would be zero-filled as Chris says. However I believe this is incorrect behaviour. It is possible to easily fix that so it would only happen via a tiny race window (where the source memory gets unmapped at just the right time) however nobody seemed to interested (just by checking the return value of fault_in_pages_readable). The buffered write patches I'm working on fix that (among other things) of course. But they do away with prepare_write and introduce new aops, and they indeed must not expect the full range to have been written to. -- SUSE Labs, Novell Inc. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org